This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Large volumes of natural gas (i.e. primarily methane) are located in remote areas of the world. This gas has significant value if it can be economically transported to market. Where the gas reserves are located in reasonable proximity to a market and the terrain between the two locations permits, the gas is typically produced and then transported to market through submerged and/or land-based pipelines. However, when gas is produced in locations where laying a pipeline is infeasible or economically prohibitive, other techniques must be used for getting this gas to market.
A commonly used technique for non-pipeline transport of gas involves liquefying the gas at or near the production site and then transporting the liquefied natural gas to market in specially-designed storage tanks aboard transport vessels. The natural gas is cooled and condensed to a liquid state to produce liquefied natural gas at substantially atmospheric pressure and at temperatures of about −162° C. (−260° F.) (“LNG”), thereby significantly increasing the amount of gas which can be stored in a particular storage tank. Once an LNG transport vessel reaches its destination, the LNG is typically off-loaded into other storage tanks from which the LNG can then be revaporized as needed and transported as a gas to end users through pipelines or the like.
Conventional plants used to liquefy natural gas are typically built in stages as the supply of feed gas, i.e. natural gas, and the quantity of gas contracted for sale, increase. Each stage normally consists of a separate, stand-alone unit, commonly called a train, which, in turn, is comprised of all of the individual components necessary to liquefy a stream of feed gas into LNG and send it on to storage. As the supply of feed gas to the plant exceeds the capacity of one stand-alone train, additional stand-alone trains are installed in the plant, as needed, to handle increasing LNG production.
In some cases, the economics of an LNG plant may be improved by driving the compressors in both a first and second compression strings through one or more common shafts. However, this does not overcome all of the disadvantages associated with each stand-alone train in an LNG plant requiring its own dedicated, compression strings. For example, a complete stand-alone train, including two or more compression strings, must be installed in a plant each time it becomes desirable to expand the LNG plant production capacity, which can add significantly to the capital and operating costs of the plant.
The rapid growth in natural gas demand has posed unique technical challenges for the LNG industry. There is a significant push towards designing and building larger capacity LNG trains. This need for larger trains requires new compressor driver and process configurations, while still reducing capital cost.
The foregoing discussion of need in the art is intended to be representative rather than exhaustive. A solution addressing one or more such needs, or some other related shortcomings in the technology would increase the efficiency and lower the cost of compressing fluids given the current state of the art.